Email Address Black Listed By Exim?

I have a customer who receives an autoresponse when someone emails his two way pager. While his email works, he does not receive this autoresponse from Nextel. I am thinking the server that Nextel uses to send &amp;email to pager&amp; autoresponses may be black listed.

Doesn't exim check a black list and deny incoming email if it is on the black list? If so, please tell me the name of the file that contains this black list.